Is it okay to ask to bring an extra guest to a casual function?

Last weekend, I experienced firsthand how this seemingly low-risk question can illicit two wildly opposite responses. The first situation was a potlock dinner at a friend's house, and it was my husband who asked me if we could invite his out-of-town friend. My first reaction was a swift, "No." Because no matter how casual a gathering may seem, you never know what the host's feelings are about having extra guests, and asking only puts her on the spot.           

Fortunately (in this case), my ...